The best Black play is to save the remainder of the Black group with , allowing to capture two Black stones for an eye. Note that White has an eye in the corner at a.
If refuses to give up the two stones, then and cuts. It is four liberties to two, so White wins this capturing race.
It is possible for White to save the corner group by playing first at , but will simply make two eyes and allow to connect outside. However, White did not get to capture anything. (If is played at , then can be played at and revert back to the solution.)
The atari at here is a mistake. After connecting at , if White cuts the group with and , then lives with two eyes.